FATE OF FERTILIZER NITROGEN AS AFFECTED BY TIME AND RATE OF APPLICATION ON CORN

Management of fertilizer N on corn (Zea mays L.) fan greatly affect th e efficiency of N use and the potential for adverse environmental effe cts. Field studies were conducted on two nonirrigated southern Minneso ta soils -a Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquo ll) and a Mt. Carroll silt loam (fine-silty, mixed, mesic Mollic Haplu dalf) -to determine the effect of time and rate of N application on re covery of fertilizer-derived N (FDN) in corn grain and stover and in t he soil. Nitrogen rates of 75 and 150 kg ha(-1) on the Mt. Carroll soi l and 100 and 200 kg ha(-1) on the Webster were applied at planting or at the eight-leaf stage as (NH4)(2)SO4 to the same plots from 1982 to 1984. Enriched N-15 was applied to separate microplots each of the fi rst 2 Sr to allow measurement of FDN. Grain yield responded to applied N in 5 of 6 site-yr, but not to time of application. Uptake of FDN in grain was increased by higher N rate in all cases, but by delayed app lication in only one site-gear. Total plant FDN recovery ranged from 3 1 to 60% at the low N rate and from 24 to 45% at the high rate. Both y ields and FDN recovery were affected by unusually drg midseason condit ions. Fertilizer-derived N recovery from the soil after harvest ranged from 25 to 56%, with a large proportion at the high N rate in inorgan ic forms, especially with the late application, which increased the po tential for leaching losses. Residual uptake of FDN by grain ranged fr om 1 to 10% of the initial N rate. The difference method for estimatin g FDN recovery gave different results from the N-15 method, emphasizin g the importance of examining both labeled and nonlabeled N pools for complete interpretation of N-15 studies.